There are known articulated vehicles, such as harvesters, which move on a terrain and perform harvesting, and which are equipped with a so-called harvester head at the end of a boom assembly for cutting and felling a growing tree stem and for sawing the stem to pieces of desired length. The sawed tree stems are collected with another known working machine that moves on a terrain, wherein the working machine in question is a forwarder equipped with a grapple and the stems are transported in its load space. There are also known combined machines in which the functions of a harvester and a forwarder have been combined, wherein the loading grapple can be replaced with a harvester head that is also suitable for loading or there are several boom assemblies in the machine.
Typically combined machines and forwarders comprise two successive frame structures, which are arranged to turn around a vertical axis and/or rotate around a horizontal axis in relation to each other by means of a frame joint. In forwarders the front frame is supported, for example, by one pair of wheels, and a cabin and a power source are placed thereon. A load space and a boom assembly are placed on top of the rear frame, said boom assembly being located between the cabin and the load space. The rear frame is supported, for example, by two pairs of wheels. The wheels can also be replaced by a bogie, which typically comprises two wheels that sway together.
If the joint allows the frames to rotate, the function in question is typically prevented during the operation when the vehicle is stopped. Thus the frame joint is locked. Such a heavy rear frame and its load are supported and receive supporting force and a support moment from the front frame as well. The supporting forces are finally affected by a wheel, which presses against the ground, or by a mass of another frame alone. Supporting forces are necessary, because the boom assembly is used to handle even heavy tree stems, which may be very far on the side of the vehicle. When the vehicle is mobile, the frame joint is released, and therefore the rear frame cannot lean on the front frame and vice versa. When moving on a terrain, the soil is considerably uneven and the vehicle must be sufficiently stable, because otherwise the unevenness of the terrain can, for example, cause the load space to sway or, in an extreme case, to fall. One critical moment is the start-off at a situation, wherein the center of gravity of the boom assembly and/or the load space has settled in an unfavourable manner and the rear frame leans on the front frame via the frame joint. When the frame joint opens, the supporting forces exit and the load space can fall or sway even fiercely, in which case the sway itself can cause the vehicle to finally fall. The fall is caused, for example, by the weight completely shifting on to the wheels supporting the load space, in which situation the terrain may yield.
A rotating frame joint is introduced in U.S. Pat. No. 4,079,955, wherein the locking is based on the use of a disc brake. U.S. Pat. No. 5,328,197 introduces a forwarder, wherein the frame joint allows rotation around the longitudinal direction and folding around the transverse direction. Rotation is locked by closing the position of cylinders by means of valves. U.S. Pat. No. 4,444,409 also introduces a frame joint of a working machine, wherein free movement is limited by means of mechanical limits. U.S. Pat. No. 4,535,572 introduces a tractor joint, in connection with which are two cylinders by means of which the rotation can be affected, in which case the volume flow coming to or leaving the cylinder is choked. PCT-application WO 00/35735 introduces a frame joint of a special vehicle, which connects two frames and which can be disconnected from one frame. The frame joint includes a cylinder, which takes care of the connection and at the same time functions as a shock absorber.
The shock absorption described above, which is typically based on the choking of air or fluid, affects the stability of a working machine, but prevents the occurrence of violent and fast sways only. The speed of a medium travelling via choking increases if the pressure difference over the choker increases. This means that even in the same rotation position of the frames the intensity of absorption depends purely on the rotation speed and dynamics of the frames, in which case there is necessarily no absorption at all with very slow movement.
During absorption the load and balance situation of the working machine can change, in which case the return to a stable state may be easier. However, if the situation has not changed, the working machine or one of its frame parts falls no matter what, but slower. The shock absorption is based purely on the speed of the absorbed movement, i.e. the rotation speed of the frame parts, but it does not prevent the movement from continuing as slow and, for example, the load space from falling. In this situation the frame joint could naturally be locked, if there is time, but the sway of masses caused by the stopping would cause the load space to finally fall. Thus, according to background art, the frame joints and their shock absorptions do not function in the best possible manner in order to a stabilize the working machine even in states of change. A continuously effective absorption may limit and slow down the normal rotation of the frame parts too much, which then can cause many load variations between different wheels and driving on terrain is disturbed.